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bbean

10k Thermistors and cRIO

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Has anyone used cRIO to measure a bunch of thermistors?  We have to add thermistor measurements to an existing cRIO chassis and I'm surprised to find out NI doesn't offer an out of the box solution for this.  From what I've seen they say you can use the NI-9219 for thermistors up to 10k but the current would be way to high and would cause self heating. 

 

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Haven't used it with a cRIO, but have with E and X series cards set up as a voltage divider.

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The absence of a thermistor module has been a huge hole in the C-Series modules since they were first introduced.  I have wanted to know what the deal is with that since the first time I needed to read a thermistor with cRIO about 13 years ago.

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Unfortunately you probably have to make a voltage divider but the Winford enginering terminal blocks are pretty useful for this because you can solder in your divider resistors and then connect the block to the module with a d-sub cable

https://www.winford.com/products/brkdd37mf.php

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Why do you say the current would be too high with a 9219? The user manual say it would apply 2.2V which would be 400uA.

I guess if you wanted a lower current you could get a current source module and measure the resulting voltage with an analog input card to get resistance.

Edited by infinitenothing

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7 minutes ago, infinitenothing said:

Why do you say the current would be too high with a 9219? The user manual say it would apply 2.2V which would be 400uA.

The thermistors we are using are very small and based on the experience of others on the project, current > 100uA causes self heating of the thermistor and sensor error.  Ideally we run @ 10 uA source current

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The NI-9219 has a programmable constant current source and +-25mA is the range, isn't it?. The examples show a default excitation current of 50 or 100uA. I don't know what the resolution is off hand so don't know if it can go down to 10uA, though.

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20 minutes ago, ShaunR said:

The NI-9219 has a programmable constant current source and +-25mA is the range, isn't it?. The examples show a default excitation current of 50 or 100uA. I don't know what the resolution is off hand so don't know if it can go down to 10uA, though.

I confess I haven't dug too deep into this but based on the support note here, I'm not to optimistic: Thermistor Measurements Using the NI-9219

  • there is a 10K upper limit for resistance measurement (we have 10k thermistors...so the resistance will exceed this at lower temperatures)
  • The NI-9219 has an unregulated voltage source that excites anywhere from 220µA to 420µA across the thermistor depending on its resistance

10k thermistor measurements seem like a pretty common item, not sure why NI doesn't have an off the shelf module/approach for this with cRIO/cDAQ

Support note Snip-->

" In cRIO and cDAQ, measuring thermistors can be accomplished by setting the NI-9219 to 2-wire 10 kΩ resistance mode. This will return a resistance measurement that can be scaled to temperature using the thermistor's scaling coefficients. DAQmx doesn't support thermistor measurements on all channels. On NI-9219 DAQmx supports channels _cjtemp0:3, but not channels ai0:3. If using this configuration, keep in mind the following limitations: 

  • The 2-wire resistance measurement configuration is easier to assemble than the half-bridge configuration, but it does not take into account measurement accuracy.
  • The NI-9219 has a 10 kΩ limit for making 2-wire resistance measurements, which is too narrow for many thermistors. This is the first limitation. 
  • The NI-9219 has an unregulated voltage source that excites anywhere from 220µA to 420µA across the thermistor depending on its resistance (the excitation values for resistance mode can be found in NI-9219 Operating Instructions and Specifications). This could potentially affect the accuracy of some thermistors, because the current may cause self-heating errors. Refer to the thermistor datasheet to determine if self-heating will be an issue. 

"

 

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1 hour ago, ShaunR said:

The NI-9219 has a programmable constant current source and +-25mA is the range, isn't it?. The examples show a default excitation current of 50 or 100uA. I don't know what the resolution is off hand so don't know if it can go down to 10uA, though.

No not programmable

Quote

Excitation level is a characteristic and is not software-selectable

I've never had a problem with self heating but I've always had really good dissipation. I think if you need a lower current, you can use the half bridge method others linked to or a programmable current source like the 9265 with a second analog input module.

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47 minutes ago, bbean said:

Currently exploring using some of these:

Had some good luck with Acromag modules the couple of times used them. Curious how these particular units work out (or don't) for you.

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